- Eric J. Hallman added an answer:8What is the physical meaning of Integrated Sachs-Wolfe effect, and how is it different to the Rees-Sciama effect?
I found some papers discussing ISW as a proof of dark energy, but what is its physical meaning? What is your opinion? And how to calculate it?
I think I didn't answer your question regarding the "physical meaning" of these effects. Physically, they are a result of the frequency shifting of CMB photons due to the change in size of the gravitational potential they are traveling through during that travel time. Meaning, the photon enters the potential, and is blueshifted traveling down into the potential well. The large size of the cosmological potential wells means that as the photon travels, and is still under the influence of the object's gravity, it can grow in mass, deepening the potential. Therefore, when the photon climbs out of the potential, being redshifted by that travel, the corresponding redshift from leaving the deeper potential is larger than the size of the blueshift it encountered when entering the potential. Therefore, on scales relevant to the size of the potentials that are important (for ISW it's large angular scales, for RS it's smaller scales), there is a distortion in the CMB that does not correspond to effects that occur at recombination.Following
- Patricia J Lampens added an answer:14Can you identify?
I am a science professor at the Lebanese university, and I have interests about astronomy physics... I have a telescope (150/1400 mm). I have observed something strange (a group of blue stars surrounded by a group of red stars),(photos attached). I believe that it is a globular cluster but I'm not sure, and I'm certain that isn't out of focus. Can you please recognize what i have observed and if it is discovered ?.
Hello, it seems to me that it is an instrumental pattern (maybe some faint stars superposed too). What type of camera did you use? To know for sure, you need to take a few calibration frames such as a bias, dark (subtraction) and flat-field (division). Only after taking out these effects, could you tell what you observed on the image. There is an additional edge effect too, which makes the halo appear asymmetric. You could consult a manual from any software application useful for this, and find some basic information on the internet , e.g. even from a company which sells CCD camera's.
For example, some definitions are given at https://www.sbig.com/astronomy/about-ccd-imaging/ccd-imaging-101/Following
- Charles Francis added an answer:3Why are there different gases for different galaxies?In different galaxies there appear different gas structures. What is the mechanism adapting certain gases in certain galaxies while excluding the other gases?
Of relevance may be that there is continuing star formation in spiral galaxies, but not in elliptical galaxies.Following
- Mohamed Th. S. Heikal added an answer:5Is it possible to find pure iron phase in meteorite without any inclusions?
In the Chelyabinsk meteorite in particular.
Iron meteorites were strongly used by ancient Egyptians as a tool in agriculture and other uses. I strongly some colleagues about pure iron associated with Ni & Co as mentioned in Kamel impact crater crater of Egypt.Following
- Manuel Morales added an answer:99+As a researcher claimed that black holes do not exist , is any one interested in finding non singular general relativity?
A researcher claimed that black holes do not exist the link is:
Is any one interested in solving the singularity problem in general relativity?
What are the suggested ideas?
Although my invite for research contributions are initially focused towards grade school children, I invite my colleagues here at RG to feel free to participate as well (see link).Following
- Mohammad Ayaz Ahmad added an answer:35What is the structure of black holes?In some books popularizing science (e.g. “Astronomy for dummies” by S.P. Maran) it is written that black holes have the following structure: falling matter, event horizon, singularity. This structure does not coincide with the classification used in special literature where the accretion disk forming by falling matter is included. Is the black hole structure in the book above an adequate explanation for non-specialists?
Really, we do not know what the inside of a black hole. Describing the characteristics of the structure of a black hole still remains one of the challenges of modern relativistic astrophysics.Following
- V. G. Kurt added an answer:11What are the popular plotting softwares that astromomers and astrophysicists use for publication? I am using XMGRACE and GNUPLOT? What about Matlab?
Supermongo and IDL are also very popular among astrophysicists. Are Matlab, Mathematica, Xmgrace, Gnuplot also popular among some groups?
I prefere MATLAB, but I know some my friends which like MATCAD. I suppose that all depend from personal experiance. For plot of results of calculation by MATLAD I preferre very simple program which made my friend Alex Berezin special for this purpose. Vladimir G. Kurt.Following
- Xiangqian Wu added an answer:7How can I calculate or where can I find the selenographic coordinate of the point where the Sun is at zenith?
I need to know the time-varying location, in terms of selenographic latitude and longitude, of the point where the line connecting the centers of the Sun and Moon intersects with the lunar surface, to the accuracy of second and kilometer, from 2000 to 2020. Thanks!
Thanks a lot!Following
- Michael Peck added an answer:5What is causing Dark Flow?
Dark flow is an astrophysical term describing a possible non-random component of the peculiar velocity of galaxy clusters. The actual measured velocity is the sum of the velocity predicted by Hubble's Law plus a possible small and unexplained (or dark) velocity flowing in a common direction.
Does Dark flow exits ? If yes what is the cause of it?
1. There are many affects due to sample dependence, analytical methods and luminosity distance; I'm in no way saying you are unaware of them. However, I do think it is important to consider that the great attractor is estimated to be 80 Mpc away relative to the extent of the dark flow (at least 800 Mpc or almost 0.2z).
2. I've only looked at articles by Kashlinsky and/or Atrio-Barandela, as there was controversy surrounding the Planck analysis by others.
3. The question was asking about the source of the dark flow, which I suppose I can offer an answer to. Consider a sink-source universe with a cosmological scale gravitation potential, i.e. no energy is created or destroyed. The simplest solution would be a continuous, but collaminated 'big bang' arising from the center of the potential (CMB -> gravitational redshift). This than forms into the locally hot x-ray clusters, various galaxies and a Hubble flow, where the bulk flow begins to fall back into the potential (the "dark flow"). Gravitational lensing will then project local geodesics towards the center; i.e. we are observing objects accelerating back into a global gravitational potential through a cosmological-scale lens. This would further explain why volume element/angular scale observations support a static metric, increased entropy with redshift (cold baryonic matter, metallicity and mergers) and hemispherical anomalies versus homogenous universe.Following
- Robert Loughnane added an answer:3Is there a database of community-available antennas - beam size and efficiencies?
Is it possible to source a URL or database of the available antennas in the field of millimeter and submillimeter astronomy?
I need to be able to tabulate available frequency-dependent beamsizes and antenna efficiencies.
True Johannes, but the beamsizes usually stated in the observing section of a specified article generally represent a frequency range. However, your point is noted. Thanks.Following
- Fatemeh Tabatabaei added an answer:7What methods are the best to measure the metallicity in the ISM?
There are several methods and observers which can be used to estimate the metallicity in the interstellar medium, most of which are based on the measurements in the HII regions. First, what methods give the most reliable estimate? Second, has there been any measurement in more diffuse ISM in the Milky Way?
The radial gradient is an observational fact (which even could have been complicated by migration of stars in some cases). In any case, the environmental effects and the star formation feedback should not be neglected.Following
- Ksh. Newton Singh added an answer:3What will be the speed of sound in quark star if we consider EOS proposed by MIT bag model for an anisotropic fluid?
If we consider MIT Bag Model EOS, the square of speed of sound for quark star should be around 0.33. But whether this is true for an anisotropic quark star or not. So whether EOS from MIT Bag Model is for isotropic or anisotropic matter or for both?
Thanks Prof. UechiFollowing
- Ksh. Newton Singh added an answer:14Does a free fall collapsing dust radiate gravitational waves?
A collapsing star when explode (supernova), due to the sudden ejection of massive mass around the central core, there is a disturbance in space-time leading to emission of gravitational waves. But what will happen if a collapsing goes on till black hole is formed, without any explosion? Will there be an emission of gravitational waves due to the continuous grow in curvature because of the growing mass?
Thank You Dr. YangFollowing
- Marshall Eubanks added an answer:27How do I determine the spin rate of solivagant exoplanets and substellar objects?
Solivagant (nomadic) planets are roaming the interstellar space. Depending on the steepness of the mass ditribution law, There may be significantly more substellar objects in the vicinity of the Sun than there are normal stars. A few nearby extremely cool object of super-Jupiter mass have been discovered (e.g., one with WISE). Despite the absence of light, such systems of planetary mass may be teeming with life. According to M. Eubanks, more solivagant planets will be observed in the future with JWST, ALMA and SPICA. My calculations show that the tidal heating of Earth generated by the Moon may presently come up to ~5 TW. A heat source of this order can sustain a massive subsurface ocean on a lonely exoearth for gigayears. The question is, how to observationally verify that nearby solivagant planets rapidly rotate? The spin rate of some stars has been determined photometrically from the modulations caused by persistent features (dark or hot spots) on the photospheres. Would that be the best way to observe the spin of very cold planets? Are there other possibilities?
Most galaxies, projected on the sky, are elliptical, and so there are a number of papers dealing with lens ellipticity (oblateness) for strong lensing (see below). I do not think there will be much difficulty in introducing this into the microlensing formalism. Whether or not the data will be good enough to actually determine nomad rotation oblateness is, of course, another question.
- Markiyan Semenovich Chubey added an answer:40Is there a future for ground-based astrometry after GAIA?Considering the revolution that occurred in terms of optical astrometry due to astrometric satellites such as Hipparcos (even though there were parallel developments and major improvements on ground-based astrometric telescopes), an even larger jump is about to occur with the GAIA astrometric mission. Much work will need to be done to tie the radio reference frame (ICRF2) to the GAIA optical reference frame. There will be ground-based follow-up work following GAIA detections, proper motions and parallax work, as satellite missions are relatively short lived and expensive. But, what does the future hold for ground-based astrometry? Near Earth objects? Solar system measurements? Reference frame maintenance?
Greetings to all.
The microarcsecond astrometry after Gaia in reality rather will be on the level of 0.05 milliarccecond because of the correlation between the parameters of the movement of technical systems (space craft vibrations, orbital and rotational movement, parameters of the instrument and its registration system, timing and pure astrographyc fits and probably some others sources, that I cannot foreseen). We must accept the opinion of David Dunham about observations of the Solar system bodies. Including, surely, the ground-based astrometric observations.
The real problem does exist in the fitting (rather recreation) of the Celestial coordinate frame, the movements of the celestial bodies in which the real theories of baricentric movements are traditionally been constructing on the base of the laws of the mechanical energy conservation. There are plenty of the work in that area for which the ground-based astrometry will be required.
It is reasonable to wait the appearance of the other space projects for detalization of the hardly solving question in Gaia project and independent other questions. For instance, involving or prolongation of the Gaia's results into diapason of the more faint stars. See, for instance, Baltic Astronomy, 24, 84–91, 2015.Following
- Marcelo Negri Soares added an answer:5What is antigravity?I want to know the basics.
Anti-gravity is an idea of creating a place or object that is free from the force of gravity. It does not refer to the lack of weight under gravity experienced in free fall or orbit, or to balancing the force of gravity with some other force, such as electromagnetism or aerodynamic lift. Anti-gravity is a recurring concept in science fiction, particularly in the context of spacecraft propulsion. An early example is the gravity blocking substance "Cavorite" in H. G. Wells' The First Men in the Moon.
In Newton's law of universal gravitation, gravity was an external force transmitted by unknown means. In the 20th century, Newton's model was replaced by general relativity where gravity is not a force but the result of the geometry of spacetime. Under general relativity, anti-gravity is impossible except under contrived circumstances. Quantum physicists have postulated the existence of gravitons, a set of massless elementary particles that transmit the force, and the possibility of creating or destroying these is unclear.
"Anti-gravity" is often used colloquially to refer to devices that look as if they reverse gravity even though they operate through other means, such as lifters, which fly in the air by using electromagnetic fields.Following
- Alexander Chepick added an answer:3How can we explain Tifft's quantization of galaxy redshift?The reports by Tifft on quantization of galactic redshift are well-known to astronomers. Read for example http://www.vixra.org/abs/1309.0011. See also a recent review on redshift theories by Marmet at http://www.marmet.org/cosmology/redshift/mechanisms.pdf
I don't have articles on Tifft's quantization, but I will send you two of my articles on static models.
I think that the space of the Universe is not expanding. Test Tifft's quantization will show it, because in the framework of GR the distance between galaxies previously had to be less than it is now. Therefore, in GR dependence should be approximately inversely proportional to z.Following
- John Houghton added an answer:22Proton- proton reaction in star center can anyone help?The proton-proton reaction in star center goes on for billions of years. But when the reaction starts on the surface as in the case of the nova, it only lasts for a few weeks. Can somebody explain this difference?Following
- William Dean Pesnell added an answer:5How can we analytically calculate the Hurst exponent for a periodic function?
Let's focus on a sin(x) function. I tried using the DMA (changing sums into integrals) and the series width w(l) from Katsev & L'Hereux, Computers & Geosciences 29 (2003) 1085–1089. In the first case I got stuck with some crazy functions, and in the second (expanding logs in time series to first order as l<<T) I got... H= -1/2. I want to precisely understand why H=1 for strict periodicity.
An R/S analysis compares the properties of the time series over many different time scales. The slope of the curve then approximates the Hurst exponent. If you analyze a series with noise and a sine curve you see a discontinuity in the slope at the period of the sine curve (the Suyal et al. 2009 paper has some examples). In the absence of variations at longer timescales, such as trends, there is no information above that period to derive a slope. For the sunspot number we see a discontinuity at 11 years, just as expected, but there are longer-term signals to continue the R/S analysis at longer periods. That gives an estimate of H for the entire time series. A linear trend indeed has H=1 because the variation over a time bin is comparable to or larger than the noise within that time bin.Following
- Z. Osmanov added an answer:7Does anyone know what is the minimum level for detectability of PeV photons by modern telescopes?
One of the important questions concerning this topic is to know sensitivity of instruments - the minimum detectable flux.
Dear Dr Samvel Ter-Antonyan, thank you for the useful references
- Gerro Prinsloo added an answer:19How can we compute solar position at a given place on a given day and time?I have GPS obtained UTC time (hours, minutes, seconds), longitude(deg E), latitude (deg N) and date. I have thoroughly search on internet for step-by-step procedure to obtain solar position variables - solar zenith angle, solar azimuth angle, Sun-Earth distance. But every method is different. Some followed geometrical method while most others have some complicated formulae with varying coefficients. I never found a generalized way to obtain solar position variables. Is there any reference which provides step-by-step procedure to obtain them in the most accurate way? Can anyone provide the step-by-step procedure with equations, corresponding explanation for coefficients, accuracy of output and literature references for each equation? Please don't provide me readily available codes / functions or links on internet search.
you already have many responses, but if you require open source algorithms for sun tracking with matlab simulink or PLC or microprocessors then you can also check Chapter 3 of our free eBook for links to the code:
- D D Pawar added an answer:5What is the use of dark energy in f(r,t) theory?
Can we get any help of dark energy momentum tensor in f(r,t) theory?
Thank You So much...for the valuable answers!!Following
- Vikram Zaveri added an answer:22Can we use Kepler's third law to calculate orbital period of a star in a galaxy?
Kepler's third law yields correct orbital periods for the planets of the solar system
however, orbital period of the Sun in the Milky Way is computed with the relation P = 2*pi*r/v. Analysis given in the article "Supplement to periodic relativity" shows that we can obtain same result by introducing proper time in the form of deviation factor into Kepler's third law. This deviation to flat Minkowski metric satisfies Einstein's field equations and also provides solution to rotation curves of galaxies.
Following article is now published in "Progress in Physics"
Zaveri V.H., Periodic relativity: deflection of light, acceleration, rotation curves. Progress in Physics, 2015, v.11(1), 43-49.Following
- Hassan Sedaghat added an answer:21Do dark matter and dark energy constitute valid evidence of large spatial dimensions higher than 3?There are speculations that the gravitational effects of matter in 4 or greater LARGE spatial dimensions might account for the substantial discrepancy that exists between measured gravitational effects on normal baryonic matter and the amount of that matter that exists according to measurements. Could we be measuring the gravitational effects of "normal" matter in higher dimensions? And can large higher dimensions also explain the huge amount of dark energy that seems to be around?
Thomas, thank you for sending the second chapter of your book. The correlations idea adds another angle from which to look at this issue.Following
- Richard Gauthier added an answer:99+Why are mainstream physicists against the super luminal physics?
Why are mainstream physicist generally against theories, which describe faster than speed motion or communication? Is it proven in experimentally that it cannot happen or is it just a bias based on reputation of the existing theories? There are 2 very good papers about faster than light relativity listed at the bottom. Has someone refuted them on specifics?
The papers are:
It is clear that you are a materialist and that you think that you know what matter is--the fundamental substance of the material world, and the material world is the only real world for you. You are welcome to your circular opinion, but please do not think that it is logical, or scientific. There is no scientific reason to think that the substance of matter is matter. Substance is that on which matter stands, and that substance does not have to be material-- it may be energy, it may be consciousness, it may be something non-material. Matter is a concept used to help unify our sense impressions, which are surely mental and not material. And concepts are mental also.
with best wishes,
- Fabio Salvaggio added an answer:4What kind of camera do I need to do transit photometry?
I am working with a small group of students at Hamline University on observing an exoplanet using transit photometry.
Right now, our group is working on buying equipment. The school already has Celestron C-8 telescopes with motorized mounts available for us to use, but we don't have a camera. We need a camera that we can use to take a long-exposure photograph of a single star, and it needs to be able to measure the brightness of the star very accurately.
Our budget is about $1,000, but the cheaper the better. So far, the only camera under $1,000 I have found that might work is the Atik Titan. One person I asked about this camera said that the anti-blooming technology might cause problems, and that the chip in the Atik Titan is too small. This person proposed the ST-402ME as a better model, but it costs $1,500.
Does anyone know either of these cameras would be sufficient for our project? Is it worth the extra money to buy the ST-402? Or do you have any other recommendations?
Many thanks in advance for any help!
I always used SBIG ST cameras. ST7 and ST8. I'm pretty sure you can find them as used with a good price (ST7 for sure). With these cameras I achieved great precision (better than 0.002) with a C9.25 at f/10. You can see in my blog (http://fsalvaggio.blogspot.com translate it and go in exoplanet category) and in some paper I published.Following
- Patrice Poyet added an answer:29Telescope for amateur astronomy.I am a researcher at a science education project in Palestine and I coordinate for an informal science program. Many of the students I work with are interested in astronomy and space. I would like to purchase a telescope for amateur astronomy so as to maintain their interest and have astronomy-related activities at our center. I have been researching for the best amateur astronomy telescope and I have asked for advice, but I would like to hear your thoughts as experts in astronomy. One of the telescopes I am currently looking at is Celestron's CGEM - 1100 Computerized Telescope, what are your thoughts on this one? I truly appreciate any feedback / advice you may provide.
I would strongly recommend to make it as it will teach your young students a lot of things and make them share a great adventure. The book from Texereau is perfect for that http://www.willbell.com/tm/tm3.htm
Would you like to have a look at the telescopes I built myself got to:
- Dmitri Martila added an answer:12Which experiment can determine if our universe is hologram or not?
Which experiment can determine if our universe is hologram or not?
Is it confirmed yet or still is it an open question?
Perhaps you find it amusing: "Entropy of Real Pendulum" http://vixra.org/abs/1311.0045Following
- David Iain Pontin added an answer:1What are the fan and spine reconnections in the 3D magnetic reconnection of the solar corona?
How do the fan and spine reconnections take place?
This terminology refers to magnetic reconnection in the vicinity of a three-dimensional magnetic null point (a point in space at which the magnetic field strength is zero). Electric current sheets can form in various configurations around these points in response to different external forces, leading to different types of reconnection. A summary of the properties of these types of reconnection, as well as references to various articles with further details, can be found in the attached review.Following
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